This invention relates generally to the field of wireline back-off tools which detonate an explosive charge for creating a jarring force to release a selected threaded pipe connection in a pipe string.
During the drilling of a well, the drill string may become stuck in the well making it impossible to remove the drill string by pulling and/or rotating the string in the usual manner. Since a drill string is made up of multiple sections of drilling pipe which are joined together with a threaded connection formed at the mating ends of the pipe or with the use of a coupling at the threaded ends, in some cases, the upper portion of the drill string above the section of pipe which has become stuck in the well may be unthreaded from the stuck portion so that at least the upper portion of the drill string may be pulled out of the well. Since the threaded connections joining sections of a drill string are usually tightly connected, the release of one of such connections in the pipe string has been accomplished by applying a back-off torque to the pipe string and detonating an explosive charge adjacent the threaded connection to be released. The shock of the explosion serves as a jar to cause the threaded connection to release by the action of the reverse torque. Such an apparatus and method is disclosed in U.S. Pat. No. 2,305,261 issued on Dec. 15, 1942 to M. M. Kinley. Other apparatus has been disclosed with drill collar locators for positioning an explosive device adjacent a drill collar. Such apparatus is disclosed in U.S. Pat. Nos. 1,560,815, 3,157,119, 3,180,261 and 3,374,735.